More than 18,000 scientists, campaigners and donors opened a major AIDS conference in South Africa on Monday, hailing progress against the disease but warning that recent gains were under threat.

The week-long conference returns to Africa 16 years after Nelson Mandela galvanised the world to take up the fight against AIDS, describing it as "one of the greatest threats humankind has faced".

Again hosted by the coastal city of Durban, the International AIDS Conference is seen as the key biannual gathering of experts tackling a pandemic that has claimed more than 30 million lives in 35 years.

Among those attending this 21st edition are UN Secretary General Ban Ki-moon, activists including singer Elton John and actress Charlize Theron, and Britain's Prince Harry.

"When the global AIDS community last met (in Durban) in 2000, only a few thousand Africans were receiving antiretroviral drugs," billionaire philanthropist Bill Gates said in a speech on the eve of the opening session.

"Today, more than 12 million Africans are on treatment -- more than a quarter of them living here in South Africa."

But other statistics are less encouraging.

In sub-Saharan Africa, more than 2,000 young people under the age of 24 are newly infected every single day, and almost half of the people living with HIV are undiagnosed.

"If we fail to act, all the hard-earned gains made in HIV in sub-Saharan Africa over the last 15 years could be reversed," Gates warned, pointing to Africa's huge young population entering the risky age for catching the disease.

Ban Ki-moon also warned that urgent action was needed to avoid progress being lost.

"As a global community, we must move quickly and decisively towards achieving the targets that will help us finally bring this epidemic to an end," he said on Monday.

The United Nations has set a target of ending the AIDS pandemic by 2030.

Infection rates are rising in many regions of the world, with Russia especially hard hit.

There are some 36.7 million people worldwide living with HIV/AIDS, mainly in sub-Saharan Africa.

Of these, only 17 million are receiving treatment.

"This conference comes at another crucial time in the HIV epidemic," said Chris Beyrer, president of the International AIDS Society.

"If we don't make the right strategic choices, we risk reversing hard-won gains. Delay is tantamount to defeat."

Until 2000, only wealthier countries had hosted the AIDS meeting.

That all changed when South Africa became the first developing country -- and the first with an unbridled epidemic of HIV in its population -- to take the helm.

The conference was a highly-charged affair, but its emotional impact transformed the AIDS campaign.